Lithographic printing processes often are used to prepare printed articles such as papers, magazines, films and labels. For lithographic printing processes, all types of lithographic printing inks known in the art can be used, depending on the selected process. These include, e.g., sheetfed, heatset and coldset lithographic printing inks.
Sheetfed printing inks normally dry as a result of an oxido-polymerization process. The oxygen surrounding the ink film reacts with the free C═C double bonds present in the ink system, which allows the formation of a high molecular weight polymer. The kinetics of this process are very slow and oxidative drying could last anywhere from several days to weeks depending on the ink formulation. Thus, in order to expedite further processing, such as cutting, folding and/or binding, the printed substrate often is overprinted with varnish. This is an added step that adds to the cost of the process.
The kinetics of oxidative drying can be increased by including metal drier or metal catalyst, which allows the ink film to be dried anywhere from several hours to 1 or 2 days. However, printing inks containing metal catalysts or metal driers have two major drawbacks. Firstly, the sheetfed printing inks are prone to premature polymerization on the press, which can necessitate time consuming and costly frequent press cleaning, or premature polymerization in storage containers, usually in the form of skin formation, which leads to sheetfed printing ink wastage and possible contamination. Secondly, the metal driers or metal catalysts are generally not user-friendly due to associated toxicity issues. Cobalt salts are the most widely used metal driers or metal catalysts in sheetfed offset printing but are regarded as toxic to handlers.
In the literature, there is no description of sheetfed offset inks that can polymerize (dry) quickly enough for further processing without the need for overprinting or including metal driers or metal catalysts. Thus, a need exists to improve the setting and drying properties and rub resistance of sheetfed lithographic inks while minimizing or eliminating metal driers or metal catalysts in the printing inks and the associated health risks and/or eliminating the need to overprint with varnish prior to further processing.
Lithographic printing inks used in heatset printing processes dry under the influence of heat, generally via solvent evaporation. The evaporation of solvent can be a major factor in the drying speed of the heatset printing ink. Heatset printing inks often include oils, such as, for example, mineral oils, which oils evaporate under the influence of heat, thereby allowing the printing ink film on the printed substrate to dry. The oils, e.g., mineral oils, can have a boiling range above 200° C. For example, some heatset printing inks often include oils have a boiling range of 220° C. to 320° C. Because the heatset printing ink does not need to have its components absorbed by the substrate, e.g., paper, in order to dry, substrates having smaller pores and/or having a coated surface can be printed.
Solvent evaporation as a drying mechanism can be accomplished at room temperature given enough time. Stacking of fresh prints in order to conserve space, which is typical during long printing runs, frequently limits the amount of oxygen available to the printed sheets and thus reduces the oxido-polymerization mechanism for drying, thereby limiting the efficacy of room temperature drying of stacked prints. To meet the efficacy requirements of the modern printer, a thermal process to accelerate solvent drying is required in heatset printing applications. The requirement that heatset printing inks be dried, such as by using a thermal process or other drying devices, means that the printing process can be energy intensive. Additionally, the incorporation of some solvents in lithographic printing inks can result in undesirable distortion, e.g., capillary spread prior to the ink drying, impacting print resolution. Thus, a need exists to improve the setting properties and drying properties and rub resistance of heatset printing inks while reducing the temperature needed to cure the heatset printing inks or eliminating the need to heat the heatset printing ink altogether.
Printing inks used in coldset printing processes dry by absorption of ink components into the substrate, such as paper. Coldset printing inks can include special auxiliary substances, e.g., surfactants, waxes, gelating agents and mineral fillers, which enhance or enable the ink film on the printed substrate surface to dry by causing ink components to be absorbed into the substrate. The printed ink film generally does not have the same strength as the film produced from a heatset printing ink formulation. Rub off can be a problem for some coldset printing ink formulations. Thus, a need exists to improve the setting properties and rub resistance of coldset printing inks.